The disclosed invention relates in general to an optical relay utilized in a spectroscopic instrument that uses a slit to define an object for a subsequent dispersive element. More particularly, this invention relates to such a relay having a relatively flat power throughput over a selected range of wavelengths. In general, an optical relay is a device that produces an image at one point from a source at another point. Such devices typically utilize lenses and/or mirrors to produce the image.
In a spectrophotometer, light is passed from a light source through a sample cell containing a sample that absorbs some of the light. The light transmitted through the sample cell is directed to a detector for measurement of the spectral intensity distribution of the transmitted light. If the range of wavelengths over which the spectral intensity is measured includes strong absorbance peaks that characterize the sample, then that sample can be identified by the measured spectral distribution. Among the sample substances that are of general interest are: (1) aromatic molecules that absorb at 230-300 nanometers; (2) amino acids, sugars and hydrocarbons that absorb strongly near 190 nanometers; (3) iron, copper, urea and hemoglobin that absorb near 820 nanometers; and (4) enzyme cofactors such as AND that absorb near 400 nanometers. Therefore, a general purpose spectrophotometer should measure the absorption spectrum of a sample over a range from about 190 nanometers to about 820 nanometers.
Unfortunately, over the wide range of wavelengths from 190 to 820 nanometers, simple lenses exhibit a significant amount of chromatic aberration. If the wavelength range only extended over the visible light range, then the fractional change in the focal length f of a simple lens would be on the order of 0.016. However, over the range from 190 to 820 nanometers, the fractional change in f is on the order of 0.2. Therefore, over such a range it is common to use either achromatic lenses which have typical fractional changes in f on the order of 0.0004 (over the visible range) or to use focussing mirrors. However, both achromatic lenses and focussing mirrors are significantly more expensive than simple lenses
Simple lenses have been used in filter photometers because such photometers utilize a wide area detector that is insensitive to the chromatic aberrations of simple lenses. In such an instrument, the wavelength is selected by placing a suitable filter in the path of the optical beam. Such instruments have the advantage of low cost, but are unable to scan an entire spectrum.
In order to scan an entire spectrum in an instrument with a dispersive element such as a diffraction grating or prism, a slit is required to define the area of the beam to be detected. In one type of spectrometer, the beam passes successively through the dispersive element, the slit and then the sample cell. In such a spectrometer, only the portion of the spectrum that is incident on the slit passes through the slit to the sample cell. The width of the slit determines how wide a portion of the spectrum passes through the sample cell so that the slit width determines the resolution of the spectrometer. The spectrum can be scanned by rotating the dispersive element to sweep the spectrum across the slit. In another type of spectrometer, the beam is passed successively through the sample cell and a slit before being incident on a dispersive element such as a diffraction grating. A photodiode array is used to detect the entire spectrum in parallel, thereby increasing the speed of operation. The width of the slit again determines the resolution of the spectrometer.
When a simple lens is utilized in either of these two types of systems over a wide wavelength range, the spectral distribution of light transmitted by the slit exhibits a peak at the wavelength focussed onto the slit. The reason for such a peak can be seen as follows. Because of chromatic aberration, the light at wavelengths other than at the distribution peak are out of focus at the slit. This results in a beam width at the slit that is wider for the out of focus wavelengths than for the wavelength focussed onto the slit. The slit width is typically selected to be on the order of the beam width of the focussed light in order to maximize resolution. As a result of this, part of each of the out of focus wavelengths is blocked by the edges of the slit, thereby decreasing the amount of light reaching the detector at these wavelengths.
For a light source having a relatively flat spectrum over the wavelength range from 190 nanometers to 800 nanometers, the spectral distribution of the light passing through the slit has the shape shown in FIG. 1. When a broad range of wavelengths is used, such as from 190 nanometers to 820 nanometers, this peaking is particularly severe. If the slit width is increased to reduce this peaking, then the resolution is decreased.
Therefore, wide band spectrometers typically utilize achromatic lenses or mirrors to focus the optical beam. These mirrors are typically manufactured by single-point machining or electroforming. Because these mirrors typically have the shape of a conical section generated by rotating a conic about its axis, they are relatively expensive to produce. In an optical relay utilizing mirrors, the mirrors are tilted at a small angle relative to the light incident on each of such mirrors so that the reflected light is not collinear with the incident light. This is done to avoid blocking part of the beam by the mirrors themselves. Such deviation of the beam path requires complex mounting surfaces produced by additional machining operations to ensure that the mirrors are rigidly and accurately positioned to accurately direct and focus the beam. Also, the mirror surface is often coated with a thin film for protection. This film adds cost and can degrade with time, thereby degrading performance. All of this complexity results in a cost that is significantly greater than for an optical relay utilizing a simple lens.
The complicated mounting and machining operations can be avoided by utilizing achromatic lenses (also called achromats) in an in-line optical design which can be quite compact. Unfortunately, if an achromatic lens is corrected for the ultraviolet portion of the spectrum extending down to 190 nanometers, then such lenses are very expensive. Because most elements are opaque to ultraviolet light, such lenses are typically made of silica and calcium fluoride and the individual elements of the achromat have powers that are much larger than the net power of the achromat. This is the reason for the high cost. The high powers of the achromat components also introduce residual monochromatic aberrations that limit use to relatively small numerical aperture or small dimensions. Achromats perform well, but are typically twice as expensive as an equivalent mirror. They are generally used where compactness is required.
The variation in index of refraction n over the range of the spectrometer produces a variation in the focal length of a lens over this range. A measure of this variation is the normalized longitudinal chromatic aberration defined to be df/f.sub.R where df is the total variation in f over the wavelength range of interest and f.sub.R is the focal length of a reference wavelength, typically selected within the range of interest. For an achromatic lens, the normalized longitudinal chromatic aberration is on the order of 0.0004. For a simple lens, the normalized longitudinal chromatic aberration is on the order of 0.016 over the range of visible light, but is on the order of 0.2 over the range from 190 nanometers to 820 nanometers. In order to reduce the expense of optical relays, it would be advantageous to have a relay design that only utilizes simple lenses, but produces a relatively flat spectral transmission over the full range from 190 nanometers to 820 nanometers.